The invention generally relates to a ceramic material for use in various coating techniques, such as for example the different variants of thermal spraying. The invention also relates to a method for producing a ceramic material of this type. Furthermore, the invention relates to a use of the ceramic material and to a layer made from the ceramic material on a metallic or ceramic body.
Different variants of thermal spraying are flame spraying, plasma spraying, high-speed spraying, detonation spraying, and coating by way of laser or powder plasma weld surfacing. These methods are used to coat highly stressed components which are exposed to abrasive or erosive wear, corrosion, high temperatures or a very wide range of combinations of such loads. Components of this type are used, for example, in automotive engineering, in mechanical engineering, in power engineering, in chemical or petrochemical installations and numerous other sectors of the economy.
In the thermal spraying method, meltable material, such as metal or ceramic, is softened or melted by heating and is applied onto a surface which is to be coated. The heated particles of the material come into contact with the surface, on which they cool and, as a result, adhere to the surface. The heating of the material to be sprayed, including the acceleration of the heated material toward the surface, usually takes place in a spray gun for thermal spraying. The material to be heated is fed to the spray gun in powder form. The mean grain size of a powder of this type is usually between 2 xcexcm and 150 xcexcm.
The powder is accelerated in the spray gun by way of a gas stream. This gas is generally one of the operating gases of the spray gun, which generate the combustion or plasma flame in the spray gun. For a plasma spray gun, operating gases of this type are usually nitrogen or argon, on the one hand, and helium, on the other hand. In this case, the nitrogen or argon simultaneously serves as carrier gas for the powder.
Various ceramic materials, or materials which resemble sintered carbides, are in widespread use as coating powder for thermal spraying in engineering. A ceramic material is used in particular if the component to be coated is to be protected against corrosion or thermal influences. One example of such an application is that of protecting against wetting by metallic or oxidic melts. Particularly with components of this type, the problem arises that high mechanical stresses are generated in the ceramic coating under thermal loads. These stresses readily lead to cracks in the coating or to the coating becoming detached from the coated component. Mechanical stress occurs when the coefficient of thermal expansion of the ceramic coating material differs significantly from the coefficient of thermal expansion of the material of the component. Therefore, it is preferable to select a material for thermal spraying which has a coefficient of thermal expansion which is similar to that of the material of which the component to be coated consists.
To coat a metallic component, it is particularly appropriate to use a ceramic material whose coefficient of thermal expansion is close to that of the metal. Since metals generally have a coefficient of thermal expansion which is greater than 10*10xe2x88x926/K, only a few oxides may be used for coating purposes. A preferred spraying material is zirconium oxide, which is used with a stabilizing additive of 7 to 9% by weight of yttrium oxide, for example in internal-combustion engines. The coefficient of thermal expansion of zirconium oxide layers of this type is in the region of 11*10xe2x88x926/K. However, the resistance of zirconium oxide to attack from metallic or oxidic melts is lower than that of a number of other materials.
MgO has a satisfactory resistance to melts, and its coefficient of thermal expansion of 13.6*10xe2x88x926/K means that it is also a suitable coating material for metals. However, MgO is not a suitable material for use in a thermal spraying process, since MgO decomposes at the high temperatures which occur in such processes, and the decomposition products are volatile.
Ceramics which are produced from a mixture of MgO and Al2O3 have good properties for use in combination with various metals. Sintered ceramics produced from MgO and Al2O3 are commercially available. They have the advantages of being highly resistant to chemical, thermal and mechanical attacks and of having a coefficient of thermal expansion which lies in the region of 11*10xe2x88x926/K. However, ceramics of this type have only limited suitability as coating material, since in practice they are not suitable for coating by way of a thermal spraying method. In these ceramics too, the MgO of the ceramic evaporates at the high temperatures which occur during thermal spraying.
It is an object of the present invention to provide a stable ceramic material which is suitable for a coating operation by way of a thermal spraying method and which has a coefficient of thermal expansion which is matched to a metal.
A further object of the present invention is to provide a method for producing a material of this type. A further object of the present invention is to describe a use of the ceramic material. Furthermore, the invention has an additional object of providing a layer on a metallic body, which is able to withstand thermal loads.
The first object is achieved by a ceramic material which, according to the present invention, includes 10 to 95% by weight of MgAl2O4, 5 to 90% by weight of MgO, up to 20% by weight of Al2O3, remainder standard impurities, and which has grains of MgO with a mean diameter of 0.1 xcexcm to 10 xcexcm that are embedded in a matrix of MgAl2O4 in spinel form.
The present invention is based on the consideration that, unlike in a sintered ceramic including MgO and Al2O3, in which MgO and Al2O3 adjoin one another, in a compound formed from MgO and Al2O3 the evaporation of MgO during thermal spraying can be prevented or greatly restricted. An example of such a compound is MgAl2O4. This compound or ceramic has proven to be a suitable material for a thermal spraying process in a number of tests. Moreover, it is highly chemically and mechanically stable. The drawback of a ceramic of this type is its low coefficient of thermal expansion of approximately 8.5*10xe2x88x926/K, which is lower than that of most metals.
Furthermore, the present invention is based on the consideration that MgO has a coefficient of thermal expansion of approximately 13.6*10xe2x88x926/K. Therefore, the introduction of MgO into MgAl2O4 leads to an increase in the coefficient of thermal expansion of the ceramic material which forms. Depending on the amount of MgO added, the coefficient of thermal expansion can be set in a defined way and can be specifically matched to the coefficient of thermal expansion of the metal to be coated,or at least the difference between the coefficients of expansion can be reduced.
In a third step, the present invention is based on the consideration that the MgO has to be incorporated in the ceramic material in such a manner that it does not decompose or sublime in the hot flame of the spray gun used for thermal spraying. It is incorporated in this way if the material has areas of MgO which are embedded in a matrix of MgAl2O4. The MgO present inside such areas, which can also be referred to as grains, is surrounded by MgAl2O4. The MgAl2O4 is preferably in the form of a homogeneous matrix which does not include MgAl2O4 grains which have been sintered together with spaces between them, but rather includes homogeneous, pore-free MgAl2O4. This MgAl2O4 is sufficiently thermally stable to preserve the covering which surrounds the MgO even during the thermal spraying operation. In this way, the area of MgO remains enclosed during the thermal spraying, and the MgO cannot sublime or evaporate.
In the range between 0xc2x0 C. and 1000xc2x0 C., the ceramic material has a coefficient of thermal expansion of 8.5*10xe2x88x926/K to 13*10xe2x88x926/K. After the ceramic material has been applied as a layer to a metallic body, for example by thermal spraying, the material has a predetermined coefficient of thermal expansion. This expansion coefficient may differ from the expansion coefficient of the material prior to the spraying. The expansion coefficient of the material of the sprayed-on layer is matched to the expansion coefficient of the metallic component which is to be coated. This matching means that, if the component is exposed to high temperature fluctuations, there are scarcely any stresses produced between coating and coated substrate. This prevents the coating from being exposed to high mechanical loads caused by temperature fluctuations and, for example, becoming detached from the substrate or forming cracks.
The present invention is able to achieve the particular advantage that the coefficient of thermal expansion of a coating produced by thermal spraying may be matched to the coefficient of thermal expansion of the coated material. This matching is effected by suitably selecting the proportion of MgO in the mixture of substances used in the ceramic material.
By matching the coefficient of thermal expansion, it is possible to achieve a specific reduction in stresses within the layer composite material formed in this way. As a result, the resistance to thermal shocks and the layer adhesion under cyclic temperature loads may be positively influenced. Furthermore, the ceramic material according to the present invention is highly resistant to aggressive melts or basic slags, such as those which are encountered in nonferrous metallurgy. Furthermore, a coating made from the material according to the invention does not undergo any relevant aging, for example through destabilization or modification changes to the structure, even under high thermal loads. A coating made from this type can scarcely be wetted by liquid aluminum or zinc. Also, on account of its white color, it has a low radiation coefficient. Moreover, a coating made from the material according to the present invention has a high electrical resistance. It is therefore also suitable as an insulator.
In an advantageous configuration of the invention, the grains of MgO embedded in a matrix of MgAl2O4 have a mean diameter of 0.1 xcexcm to 2 xcexcm. This grain size has a particularly advantageous effect on the sprayability of the ceramic material. The MgAl2O4 is expediently in spinel form. MgAl2O4 in a structure of this type is particularly suitable for thermal spraying and is particularly resistant to chemical and mechanical attacks.
The ceramic material advantageously contains 55 to 80% by weight of MgO. Depending on the amount of MgO, a material of this type has a coefficient of thermal expansion which is between 11.4 and 11.8*10xe2x88x926Kxe2x88x921 at 1000xc2x0 C. The coefficient of thermal expansion of iron and numerous iron or steel alloys is only slightly above this range. Therefore, a material of this type is particularly suitable as a coating for such alloys.
In a further configuration of the present invention, the material additionally includes at least one oxide selected from the group consisting of CaO, SiO2, ZrO2 and Fe2O3. These materials, as additives, have beneficial effects on the materials properties of the material.
The second object is achieved by a method for producing a ceramic material in which, according to the present invention, MgO and Al2O3 as starting materials are melted to form a liquid phase, then the liquid phase is made to solidify by cooling, and the solidified phase is milled to form a powder of the ceramic material.
During the melting, MgAl2O4 and, if sufficient MgO is present in the starting materials, free MgO are formed. This MgO is homogeneously distributed in the liquid phase. When the liquid phase solidifies, areas in which MgO is preferentially present and which are embedded in a matrix of MgAl2O4 are formed. This method according to the present invention produces a ceramic material which has the advantages described above.
In an advantageous configuration of the present invention, the starting materials of the ceramic are melted in an arc furnace. A furnace of this type is particularly suitable for melting the ceramic starting materials.
The starting materials MgO and Al2O3 are advantageously homogeneously mixed with one another prior to the melting. This is achieved, for example, by the starting materials being introduced into a suspension and homogenized, and then being granulated, for example spray-dried. Mixing is also achieved as a result of the starting materials being present in the form of a powder and mechanically mixed.
In an expedient configuration of the present invention, 26% by weight to 96% by weight of MgO is used in the starting materials. The defined quantity of MgO added in the starting materials makes it possible to set the coefficient of thermal expansion of the ceramic material to be produced to a predetermined value. This value is between 8.5*10xe2x88x926/K and 13*10xe2x88x926/K. As a result, the expansion coefficient of the material in the coating may be matched to the expansion coefficient of a metallic material which is to be coated.
A further advantage can be achieved by the fact that the material powder is formed into larger powder grains by agglomeration of the powder grains. This is achieved, for example, by adding a binder to the material powder, followed by fluidized-bed agglomeration or spray drying. The grain size of the agglomerated powder is, in the method, matched to the requirements of the particular coating technology. Therefore, it may lie in a wide range from 10 xcexcm to 250 xcexcm.
The object relating to use is achieved, according to the present invention, by the fact that the ceramic material as described above is used as a spray powder for thermal spraying. The material does not dissociate or dissociate only to an insignificant extent during thermal spraying. Furthermore, the material, as a result of the thermal spraying, forms a coating which adheres securely to the component to be coated and is distinguished by particular stability when exposed to thermal, chemical or mechanical attacks. It should be particularly emphasized that the coefficient of thermal expansion of a material of this type can be matched to the coefficient of thermal expansion of the material of the component to be coated by selection of the composition of the material.
The ceramic material is advantageously used as a coating in nonferrous metallurgy which is produced, for example, by thermal spraying. A coating of this type made from the material according to the invention, in particular on a part of a tool used in nonferrous metallurgy, is particularly suitable for use, for example, in a strip-galvanizing or aluminum-coating installation, for measurement sensors, blowing lance heads or for tools used for aluminum or magnesium casting.
A further advantage of the present invention is achieved by using a ceramic material as described above for coating a surface of a component of a high-temperature fuel cell by means of thermal spraying. Components of a high-temperature fuel cell, which is operated in the temperature range between 850xc2x0 C. and 1000xc2x0 C., are exposed to high thermal loads. Moreover, components of this type come into contact with chemically aggressive operating gases of the high-temperature fuel cell. The abovementioned advantages of the ceramic material, which are also inherent to a coating made from the material, are therefore particularly relevant in a high-temperature fuel cell.
The latter object is achieved by a layer including the ceramic material on a metallic body which has a predetermined coefficient of thermal expansion, the quantity of MgO in the ceramic material being selected in such a way that the material, after coating of the body, has the same coefficient of thermal expansion as the body.
A layer of this type does not form any stress cracks even under considerable thermal loads, on account of the matched coefficient of thermal expansion, since the stress within the layer composite workpiece formed in this way is reduced to a minimum. Consequently, the layer is able to withstand thermal shocks and adheres securely to the metallic body.